Epple, M. A.Y.. Institut fuer Meereskunde, Marcus.Epple@mare.de
Jochem, F. A.Y.. Institut fuer Meereskunde, Frank.Jochem@mare.de
EFFECTS OF UV-RADIATION ON BALTIC SEA PHYTOPLANKTON
In view of decreasing stratospheric ozone concentrations over the northern hemisphere, the influence of natural UV-radiation on phytoplankton assemblages in the central Baltic Sea was assessed by use of quartz glass and spectral filters to achieve different light climates and to differentiate UVA and UVB effects. 1% irradiance depths for PAR, UVA, and UVB were 15, 4, and 1.5 m, respectively. UVA exhibited >1% of surface irradiance over 25%, UVB over 10% of the euphotic zone. UV-light reduced growth rates of phototrophic nanoflagellates and reduced whole community primary productivity by up to 70%; integrated production was reduced by 5-12%. Despite 50 times lower quantum flows of UVB as compared to UVA, UVB accounted for 1/3 of UV-effects. Synechococcus-dominated picoplankton (<2 Ám), which accounted for 60% of total chlorophyll, exhibited the lowest inhibition of 50-60%. The >100 Ám fraction (30% of total chlorophyll) dominated by filamentous diazotrophic cyanobacteria (Nodularia sp., Aphanizomenon sp.), in contrast, showed a depression of 75-90% as compared to PAR. UV-light also reduced nitrogen fixation rates by 50%. The significance of these surface-floating cyanobacteria for the nitrogen import into the Baltic Sea might have been overestimated hitherto since UV-blocking incubation vessels were used. Changes in algal pigmentation were most pronounced in photo-active chlorophylls and fucoxanthin.
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